Conventional class eight diesel trucks have muffler assemblies which are subject both to low-frequency vibration from the chassis, and higher-frequency vibration from the engine. The muffler assemblies are typically either horizontally mounted, rigidly affixed to the frame of the truck, or vertically mounted, affixed with some compliance to the truck cab or sleeper, or rigidly mounted to independent stanchions behind the cab or sleeper.
Cab-mounted vertical mufflers are very common on conventional trucks or tractors, especially those without sleepers. Such direct mounting of the muffler apparatus to the structure of the cab is disadvantageous in that the high-frequency vibration of the muffler caused by the pulsations in the exhaust gas are transmitted into the cab structure, where the skin of the cab vibrates and creates noise. This noise is an annoying and tiring low-frequency boom that can increase noise levels in the cab at the firing frequency of the engine by 10 decibels or more during acceleration or hard pulls up a grade.
Ideally, interior cab noise from structurally transmitted muffler vibration can be minimized by designing a muffler mounting assembly that isolates the cab from the vibrating muffler. Since the muffler has mass, the muffler will resonate (that is, exhibit a large-amplitude vibration from minimal input, or "amplify" the input) at a set of frequencies determined by the stiffness of the compliant members isolating the muffler from the cab, by the mass and inertia and center-of-gravity location of the muffler, and by the geometry of the mounts. The muffler motions at the resonant frequencies are called "mode shapes," or simply "modes." All these modal frequencies, the muffler will transmit vibration into the cab very efficiently.
If the parameters of the muffler mounting system are chosen to locate one or more of these modes at low frequencies, the muffler will be driven to large motions by the chassis' response to road roughness, or by the resonances of chassis components. Examples include sprung-mass rigid body modes between 1.5 and 5 Hz, frame beaming between 5 and 10 Hz, and axle hop modes between 9 and 13 Hz. The resulting large-amplitude motions of the muffler would be undesirable both because the mounts would build up heat and fail quickly, and because the muffler and associated flex systems would be subjected to high forces and the potential for mechanical failure. These "ride" phenomena typically lie below 13 Hz, as shown in FIG. 1, so the ideal muffler mounting system would have no resonant modes below 15 Hz.
An inline 6-cylinder 4-stroke diesel engine fires between 30 and 105 Hz. If the muffler mounting system parameters are adjusted to locate the muffler modes in this frequency range, the muffler will resonate at the engine's firing frequency, at which considerable pulsatile energy is present in the exhaust gas stream. This energy would be amplified by the muffler mounting system, and be transmitted efficiently to the cab structure, which would produce high-amplitude noise at that frequency inside the cab. This limitation establishes an upper bound for the acceptable target muffler mode frequency range. Note that current practice locates some of the muffler system modes above 31 Hz, and that many existing trucks with cab-mounted mufflers exhibit low frequency boom to some degree as a result.
This establishes a "target window" ranging from approximately 15 to 25 Hz, allowing for some safety margin on both ends of the window. If a muffler mounting system can be built with all relevant modes in the target window between 15 and 25 Hz, then the mounting system will not amplify ride vibration, and yet it will isolate the cab from higher-frequency vibration created by gas pulsations inside the muffler. However, designing a muffler mounting system that produces muffler modes in the target window is problematic and difficult. This difficulty arises because of the inertial properties of mufflers and geometric properties of typical mounting systems. These factors tend to produce too large a spread in muffler mode frequencies, so that if the highest mode is at 25 Hz, for instance, the lowest mode might be at 5 Hz, thus amplifying ride dynamics to an unacceptable degree. If stiffer mounts are used, so that the lowest muffler mode lies at 15 Hz, the highest mode might be located near 40 Hz, producing an unacceptable noise problem.
Current prior art mounting assemblies 10 (FIG. 2) typically include an upper mounting assembly 11 and a lower mounting assembly 11' each having a pair of bushings or springs 12, 12' formed for vibration isolation of the muffler apparatus 13 from the cab 14. Each mounting assembly includes a support bracket 15, 15', formed to strap to the inlet exhaust pipe (not shown) and the upper outlet exhaust pipe 16, respectively, and a mounting bracket 17, 17' rigidly coupled to the exterior skin of cab 14.
As mentioned above, at least two isolation bushings 12, 12' are required between the respective support brackets and mounting brackets for proper stability and vibration isolation therebetween. These bushings not only provide isolation, but varying the spring stiffness thereof also varies the natural resonance frequency modes of the mounting assembly itself.
One problem associated with this two-bushing arrangement is that it is difficult to select bushing spring rates or stiffnesses which cause the natural resonant frequencies of the muffler assembly to fall within the envelope defined by the above-mentioned "target window" (FIG. 1). Varying the spring rate not only causes the resonant frequencies to shift, but also causes the frequency spread between the resonant frequency modes to shift. Hence, finding the proper combination of frequency spread and placement of the resonant frequencies is difficult if not unattainable with the prior art mounting assemblies.
This effect is due in part to the physical arrangement and location of the bushings which determine the different vibrational degrees of freedom. As shown in the graph of FIG. 3, the standard mounting assembly resonant frequency modes (spikes in FIG. 3) are spread over a large frequency range, and lie within a major portion of the engine firing frequency. Accordingly, exciting a frequency mode of the mounting assembly, during operation of the truck, is almost assured. This ultimately causes muffler apparatus 13 to vibrate sufficiently so that the low frequency "booming" is detectable in the cab interior.